1. Field of the Invention
The present invention relates to a heat transferable label and improved adhesive composition therefor.
2. Description of the Prior Art
Prior art heat transferable labels for imprinting designs onto an article typically involve decorative laminates consisting of a paper base sheet or web coated with a wax or polymeric release layer over which a design is imprinted in ink. In transferable labels of this type, it is preferable to overcoat the ink design with an adhesive coating layer, which makes it easier to transfer the ink design from the web onto a receiving article such as a plastic or glass container or bottle.
U.S. Pat. No. 3,616,015 is illustrative of the prior art. In U.S. Pat. No. 3,616,015 a label-carrying web, such as a paper sheet, includes a heat transferable label composed of a wax release layer affixed to the surface of the paper sheet and an ink design layer superimposed onto the wax release layer. In the heat transfer labelling process for imprinting designs onto articles, the label-carrying web is subjected to heat, and the laminate is pressed onto an article with the ink design layer making direct contact with the article. As the web or paper sheet is subjected to heat, the wax layer begins to melt so that the paper sheet can be released from the wax layer. After transfer of the design to the article, the paper sheet is immediately removed, leaving the design firmly affixed to the surface with the wax layer exposed to the environment. Although heat transfer labels of the type illustrated in this reference may be employed without an adhesive coating layer over the ink design, it has been determined to be advantageous in many applications to include an adhesive coating layer over the ink design to facilitate adhesion of the transferable layer onto the receiving article.
U.S. Pat. No. 3,922,435 is illustrative of a heat transferable laminate which includes an adhesive coating layer over the ink design layer. The release layer disclosed in this reference is of the dry-type release, which unlike the wax-based release disclosed in U.S. Pat. No. 3,616,015 does not melt during transfer, but yet loses adhesiveness as the laminate is heated during transfer so that the backing sheet can be readily and cleanly peeled from the transfer ink design thus avoiding "halo effects", which can occur in wax-based release layers. The adhesive layer referenced in U.S. Pat. No. 3,922,435 is composed of two different compositions depending on the nature of the receiving article. If the receiving article is a polyolefin, the preferred adhesive disclosed is a thermoplastic polyamide, which is nontacky under normal conditions but becomes tacky during heat transfer temperatures typically of between 300.degree.-450.degree. F. The polyamide adhesive is preferably applied as a lacquer. (Col. 10, lines 55-60). It may be noted, that when the thermoplastic polyamide adhesive is used, the polyolefin bottles are pretreated by passing the bottles through a hot gas flame to pretreat, i.e. preflame, the polyolefin bottle surface. (Col. 10, line 60). When polyvinylchloride surfaces, e.g. polyvinylchloride bottles, are to be labelled, the preferred heat activatable thermoplastic adhesive may be either a polyamide or vinyl acrylic resin. The vinyl acrylic material used as an adhesive as disclosed in this reference is either a blend of a copolymer of a vinyl resin such as polyvinylbutyrate with an acrylic resin such as that formed from acrylic acid or derivatives thereof such as methyl methacrylate. (Col. 11, lines 7-14).
U.S. Pat. No. 3,516,842 discloses a heat transfer label having a wax-type release layer on a carrier web, the release layer is superimposed with a design print layer overcoated with an adhesive layer. The adhesive layer composition disclosed is composed of a polyamide preferably applied as a lacquer solution over the ink design layer. (Col. 3, lines 55-60). Thus, although the release layer in this reference is a wax-based release, the adhesive disclosed is a polyamide-based adhesive of the type referenced in the foregoing patent, U.S. Pat. No. 3,922,435 employing a dry-type release.
U.S. Pat. No. 3,984,167 illustrates a heat transferable label for decorating ceramic ware. The label disclosed in this reference is composed of a paper web overcoated with a release layer, typically a wax-based release, in turn overcoated with an ink design layer and a thermoplastic adhesive coating layer over the ink design. The adhesive composition disclosed in this reference is a solvent-based adhesive, which is applied as a thermally activated adhesive lacquer. The composition of the adhesive lacquer is referenced in Examples IV-VIII. The components of the adhesive formulation set forth in Examples IV-VI are mixtures of polyethylene, dibutylphthalate, polyvinyl acetate and solvent. The adhesive lacquer formulation disclosed in Examples VII and VIII is composed of mixtures of polybutylmethacrylate, polyglycol, dibutylphthalate, and solvent (Example VII) or the mixture of polyglycol, dibutylphthalate, polyvinylacetate, and solvent (Example VIII).
U.S. Pat. No. 2,746,877 illustrates a heat transferable laminate which also includes an adhesive coating over a composite print design layer. The release coating may be either a wax-based coating as in FIG. 2 or a dry-release type coating as shown in FIG. 1. The composition of the adhesive layer is referenced at Col. 4, line 64 to Col. 5 line 4. The adhesive is activated either by heat applied during transfer or by a suitable solvent applied to the surface of the receiving article. The only reference to specific composition for the adhesive layer is that it may be composed of pigmented resinous adhesives, such as a maleate resin, an acrylic resin, or polyvinylmethyl ether. (Col. 5 lines 1-3).
U.S. Pat. No. 3,007,829 discloses a heat transferable label for use in decorating chinaware, glassware, pottery, and porcelain ware. The heat transferable laminate disclosed in this reference includes a heat activatable adhesive coating over the transferable vitreous design layer. The heat transferable label includes a heat release coating between the carrier web and the vitreous design layer. The thermoplastic or heat activatable adhesive layer disclosed therein is composed of a temporary bonding agent for affecting preliminary adherence of the vitreous design to the article being decorated. The temporary bonding agent is a thermoplastic resinous adhesive, such as maleic modified or maleate resin, acrylic resin, vinyl resin, and polyvinylmethyl ether. (Col. 8, lines 15-28) Improved results were reportedly obtained with a modified ethylcellulose lacquer residue containing a suitable plasticizer such as chlorinated diphenyl and an ester gum. Since the heat transferable laminate disclosed in this reference is applied to porcelain or chinaware, the transferred laminate is subsequently subjected to a high temperature firing to fuse the laminate to the article. During firing, the organic components contained in the vitreous design layer as well as the temporary adhesive layer are completely consumed so that only the pigmented flux component of the vitreous design remains in tact on the article.
Prior art references are apt to include statements that adhesive compositions for heat transferable laminates may be selected from conventional thermoplastic adhesive lacquers. However, closer inspection of the prior art reveals few specific formulations actually suitable for use in heat transfer laminates. As a practical matter, formulation of suitable adhesive coatings for heat transfer laminates poses very difficult problems since the adhesive must satisfy a host of specific adhesive, heat activatable, tack, melting and film-forming characteristics at low coating weights and maintain film integrity on transfer of the laminate to an article. Although a variety of plastic bottles may be decorated through the use of heat transferable laminates, it is appreciated by those skilled in the art that the most common type of plastic bottle or container to be decorated is composed of a polyolefin. It is a recognized disadvantage, however, that when decorating polyolefin bottles with heat transferable laminates employing prior art adhesives, the bottles must be subjected to preflaming operation prior to transferring the laminate thereto. The hot gas flame used in the preflaming operation causes surface oxidation of the polyolefin material, which in turn permits a uniform adhesion of the transferred laminate during subsequent heat transfer of the laminate onto the bottle. Preflaming of polyolefin bottles has been found necessary with conventional adhesives employed in heat transfer labels suitable for transfer onto polyolefins.
The necessity of preflaming polyolefin articles when employing conventional adhesive for heat transferable laminates adds considerably to the complexity and expense of the process and limits the production rate at which these bottles may be decorated.
Accordingly, it is an object of the present invention to provide an improved adhesive coating for heat transferable laminates which eliminates the need of preflaming polyolefin articles prior to the step of decorating such articles with heat transferable laminates.
It is an important object of the present invention to provide an adhesive coating formulation for heat transferable laminates which is equally suitable for effecting transfer and adhesion of the laminate to polyolefin.
Another object of the invention is to provide an adhesive coating for heat transferable laminates which is coatable at low coating thickness on conventional release layers and ink design layers, and yet maintains its film integrity during heat transfer of the laminate to an article.
Another object is to provide an adhesive coating exhibiting highly sensitive heat activatable tackification in very short heat contact time.